1. Field of the Invention
The present invention relates generally to the field of superconducting devices such as memory cells, and more particularly to vortex-transitional (VT) superconducting memory cells.
2. Description of the Related Prior Art
Integrated circuits, such as memory arrays, should be as dense as possible and operate at high speeds. Josephson effect, discovered in 1962, can be used to make extremely fast electronic switches, generally referred to as Josephson junctions. Josephson junctions are made of superconducting material. In addition to their short switching time, Josephson junctions also have very low energy dissipation.
Various Josephson junction technologies made with low Tc materials (such as Nb, Sn, Pb), have been studied for point contact junctions, bridge junctions, micro-bridge junctions, and junctions with variation in thickness. Despite outstanding characteristic properties of these Josephson junction devices of low Tc materials, such as extremely high operating speed, low power dissipation, and high sensitivity for magnetic field detection, they are often considered to be impractical for certain applications because the low Tc superconductor starts superconducting only at an extremely low temperature close to the liquid helium temperature (4.2.degree. K.). NbN is a low-Tc material with an operating temperature of 10.degree. K.
However, with a recent discovery of superconductivity at high superconducting transition temperatures, in oxide superconductors which have their transition temperature higher than the temperature of liquid nitrogen, it is possible to manufacture practical superconducting devices made of such material. The discovery of La.sub.2-x Sr.sub.x CuO.sub.4 type oxides with Tc close to 30.degree. K. and Yba.sub.2 Cu.sub.3 O.sub.7-y type oxides with Tc close to 90.degree. K. has made it possible to make Josephson junctions which have much more convenient working temperatures, in the region of the temperature of liquid nitrogen (77.degree. K.).
Josephson junction made of high Tc material, commonly described as a weak link, is a tiny barrier separating two superconductors, and may be a dielectric barrier in the order of thickness of 50 A.degree., a tiny constriction, or a point contact. It is particularly usable for fast switching Josephson devices, such as memory cells. A highly used Josephson device is the single junction RF superconducting quantum interference device (SQUID). This device is equivalent to a superconducting ring having a single weak link coupled to a resonant circuit driven by a constant current source at a selected RF frequency. Both the Q-factor and the resonant frequency of the circuit are modified by the characteristics of the coupling to the SQUID, and dependent upon the magnetic flux through the ring.
The other highly used Josephson device is the two-junction DC SQUID in which a superconducting loop incorporates two Josephson junctions in parallel. For the DC SQUID, the maximum supercurrent across the device, the critical current Ic, is a periodic function of the magnetic flux enclosed in the superconducting loop. A DC SQUID is usually operated in a resistive mode at constant current, in which the total current is due in part to superconducting electrons and in part to normal electrons.
Josephson junctions are the basic constituent elements of numerous complex superconducting electronic devices and circuits, such as data switches, A/D converters, SQUIDS, mixers, correlators, and ultra high performance computing devices. Josephson computers use Josephson devices because of their high intrinsic switching speed and low power dissipation. Josephson devices may have other, distinct components, such as resistors, conductors, transmission lines, inductors, and capacitors, some or all of which may be made of superconducting materials.
Josephson computers preferably use superconducting Josephson Random Access Memory (RAM) architecture. In the hold state this type of RAM provides sub-nanosecond access time and zero power dissipation. The central component of the superconductive Josephson random access memory architecture is a superconducting memory cell, such as a non-destructive read-out (NDRO) latch device. The most efficient and successful conventional Josephson NDRO latch device is the Vortex Transitional (VT) memory cell, which consists of two distinct parts: the storage stage and the read-out stage, as shown in FIG. 1. In general, VT memory cell is a single flux quantum vortex transitional non-destructive read-out Josephson memory cell in which the magnetic field entering a sensing gate increases abruptly, and there is a vortex transition occurring in the sensing stage loop. The storage stage loop stores a persistent circulating current corresponding to a single flux quantum.
FIG. 1 is a schematic diagram of an exemplary conventional vortex transitional memory cell with transformer coupling between its storage 5 and read-out 6 loop and a SQUID in the read-out loop. Josephson junctions 1-4 are represented by character "X". The loops 5, 6 are operated in a current bias or resistive mode and are connected by a transformer coupling element 7. The memory cell data, saved in the storage loop 5 during the storage operation, represents binary information characterized by the presence (digital "1") or absence (digital "0") of a single quantum of magnetic flux. The "1" state corresponds to the presence of a small persistent current circulating in the superconducting storage loop 5. The read-out loop 6 utilizes a two-junction superconducting quantum interference device (SQUID).
Some conventional VT memory cells with NDRO circuit design are described in U.S. Pat. Nos. 4,130,893 and 4,601,015, in the article by W. H. Henkels entitled "Fundamental Criteria for the Design of High-Performance Josephson Nondestructive Readout Random Access Memory Cells and Experimental Confirmation", published in J. Appl. Phys., 50 (12), December 1979, pp. 8143-8168, in the article by S. Tahara et al. entitled "Experimental Vortex Transitional Nondestructive Read-Out Josephson Memory Cell", published in J. Appl. Phys. 65 (2), pp. 851-856 (Jan. 15, 1989), and in the article "High-Frequency Clock Operation in Josephson RAMs" by S. Nagasawa et al., published in Extended Abstracts, Vol 2, pp. 292-290[sic], ISEC '97.
In the devices described in these patents and articles the storage and the read-out stages of the memory cell are physically separate, and coupled magnetically via transformer coupling. However, it is well known in the art that transformer coupling limits circuit performance and operating margins, because coupling efficiency is generally only about 50%. Moreover, a specialized fabrication process is required to produce an integrated circuit with a transformer coupling element, especially when the transformer coupling element should have a high coupling efficiency.
Therefore, there is a need for an efficient superconducting cell which may be used as an element of a memory array and does not require a specialized fabrication process.